A light ray incident on a material will experience scattering when it encounters inhomogeneities in its path. These could be in the form of dust particles, local fluctuations in density, or even individual molecules (Rayleigh scattering), the general phenomenon being referred to as the Tyndall effect. The Tyndall effect can be explained in terms of classical physics and includes the proviso that the scattered light has the same wavelength as the incident ray.
In the Raman effect, which is a non-classical quantum effect, a small fraction of the scattered light is found to have a wavelength that is slightly longer than that of the incident ray. Assuming the latter to be monochromatic, a Raman spectrum, which is characteristic of the material being illuminated, can be obtained by analyzing the wavelength of light scattered from the material. The Raman lines are found to always be displaced from that of the incident ray by a fixed number of wave numbers (reciprocal of the wavelength), regardless of the incident ray's wavelength. In a solid, the Raman spectrum represents energy differences between vibrational states of the crystal lattice, making it unique with respect to that particular lattice.
To obtain the Raman spectrum, a high intensity monochromatic beam (i.e. a laser) is directed at the material under investigation and light emerging in a backscattered direction relative to that of the laser is analyzed. Although light scattered in any direction could in principle be used, the backscattered direction is preferred because it minimizes any interference by the direct beam.
The present invention is an application of the Raman effect to deal with a particular problem that often arises in the course of manufacturing integrated circuits, namely determining whether a material that has been subjected to a phase changing stimulus (most commonly a Rapid Thermal Anneal or RTA) is, in fact, in the desired phase. This can be a problem for two reasons--the amount of material involved is very small and the process window for the RTA (or other procedure designed to bring about the phase change) is very narrow.
The present invention, while of a general nature, is particularly concerned with monitoring a specific process, namely the SALICIDE (self-aligned silicide) process. As will be described in further detail below, said process ends with an RTA whose purpose is to change the crystal structure of a metal silicide to a different structure having a significantly lower resistivity. The procedure for monitoring the success (or failure) of this step, as taught in the prior art, is to measure the resistivity (or rather sheet resistance) directly. However, when the area concerned becomes very small, such a measurement cannot be performed in-line. It becomes particularly difficult when the area in question is not continuous but is made of many smaller areas, such as in a line pattern. Thus, this measurement must, at best, be made on an area adjacent to the area of interest and at worst it becomes a destructive technique, requiring the sacrifice of one of the chips of the wafer being processed.
The SALICIDE process, specifically using titanium, is described by Rastogi (U.S. Pat. No. 5,286,678 Feb. 1994) who monitors the effect of the phase transforming RTA through measurement of the average sheet resistance of the titanium silicide.
The use of the Raman effect to study the structure of a thin layer in the form of an adsorbate is described by Milne et al. (U.S. Pat. No. 5,017,007 May 1991). The principle thrust of this invention is how to effectively increase the adsorbate area, and hence the intensity of the Raman signal, by careful preparation of the substrate onto which the adsorbate will attach itself. They accomplish this by depositing a layer of silver micro needles onto the substrate. This has no bearing on the present invention where the type of surface to be Raman analyzed may not be altered.